The acetic acid has a methyl group attached (CH3), which is an electron-donating compound. As such, the anionic form of the acetic acid is less stable since the CH3 does not delocalize the negative charge and instead adds to it. Meanwhile, the H atom attached to the formic acid is electron-withdrawing, and it stabilizes the product of the acid-base reaction by delocalizing the electron densities, so the forward reaction will be more favorable and the formic acid will more readily give away a proton.

Ryan Chew 1C wrote:The acetic acid has a methyl group attached (CH3), which is an electron-donating compound. As such, the anionic form of the acetic acid is less stable since the CH3 does not delocalize the negative charge and instead adds to it. Meanwhile, the H atom attached to the formic acid is electron-withdrawing, and it stabilizes the product of the acid-base reaction by delocalizing the electron densities, so the forward reaction will be more favorable and the formic acid will more readily give away a proton.

so formic acid is more acidic because the conjugate base is more stable?

Oh I think I got it. It's because of the CH3 in acetic acid is electron donating, and it contributes e- density towards the O-H bond making the bond stronger to break and H+ ion less wanting to go and attach to another compound.